i had (always have) winter tires, even with studded snow tires i had the wiggle, but you are right it is road conditions and speed, short wheel base cars have more pitch radius (spin?) feel than long wheel base cars, its a stability issue. Lowering your car makes it more stable and the sway bar stiffened the rear end (good in this aspect, bad in some others).

if you either let off the gas a little, to coast the car the wiggle will go away. Like i said mine only did it in 40-50mph range, i'm sure stock height would continue to sway at higher and maybe even lower speed.